Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: Iee16ef748ba15d7cb1d2bf3642fad57de30c8134

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.3.1
+minversion = 3.18.0
 envlist = py38,pep8
 skipsdist = True
 
@@ -16,7 +16,7 @@ deps =
        -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-whitelist_externals = sh
+allowlist_externals = sh
                       find
 commands = find . -type f -name "*.py[c|o]" -delete
            stestr run {posargs}